Fixing device and image forming apparatus including the fixing device

Abstract

A fixing device for fixing an image on a recording medium includes an endless fixing rotary member which is formed into a loop and comes into contact with the image on the recording medium, an opposed rotary member which is in contact with the fixing rotary member, a nip forming member provided inside the loop of the fixing rotary member to be in contact with the opposed rotary member via the fixing rotary member to form a nip portion to which the recording medium is fed in a feeding direction, a support member which supports the nip forming member, and a heating source which heats the fixing rotary member. The nip forming member includes a downstream portion extending downstream in the feeding direction from a center of the nip portion and an upstream portion extending upstream in the feeding direction from the center and longer than the downstream portion.

What is claimed is: 
     
       1. A fixing device which fixes an image on a recording medium, the fixing device comprising:
 an endless fixing rotary member formed into a loop to come into contact with the image carried on the recording medium; 
 an opposed rotary member to contact the fixing rotary member; 
 a nip forming member inside the loop formed by the fixing rotary member to be in contact with the opposed rotary member via the fixing rotary member to form, between the fixing rotary member and the opposed rotary member, a nip portion to which the recording medium is fed in a feeding direction, the nip forming member including a downstream portion extending downstream in the feeding direction from a center of the nip portion and an upstream portion extending upstream in the feeding direction from the center of the nip portion and longer than the downstream portion; 
 a support member to support the nip forming member; and 
 a heating source to heat the fixing rotary member, 
 wherein a gap extending parallel to the feeding direction between an edge, which is perpendicular to the feeding direction, of an upstream end portion of the nip forming member and an inner circumferential surface of the fixing rotary member is less than a gap extending parallel to the feeding direction between an edge, which is perpendicular to the feeding direction, of a downstream end portion of the nip forming member and the inner circumferential surface of the fixing rotary member. 
 
     
     
       2. The fixing device according to  claim 1 , wherein the heating source heats the fixing rotary member where the fixing rotary member is away from the nip portion. 
     
     
       3. The fixing device according to  claim 1 , further comprising holding members configured to rotatably hold the fixing rotary member and regulate the axial position thereof by contact with lateral end portions of the fixing rotary member. 
     
     
       4. The fixing device according to  claim 1 , wherein the nip forming member guides a portion of the fixing rotary member other than lateral end portions of the fixing rotary member. 
     
     
       5. The fixing device according to  claim 1 , wherein the nip forming member comprises a base pad which determines the shape of the nip portion, and
 wherein the base pad comprises
 a contact portion which is in contact with the opposed rotary member via the fixing rotary member, 
 an extended portion which is not in contact with the opposed rotary member and extends upstream in the feeding direction from the contact portion, and over which the fixing rotary member slides, and 
 a curved portion which is not in contact with the opposed rotary member and is provided to smoothly continue upstream in the feeding direction from the extended portion. 
 
 
     
     
       6. The fixing device according to  claim 5 , wherein the fixing rotary member is not in contact with the curved portion of the nip forming member in a state in which the opposed rotary member is separated from the fixing rotary member. 
     
     
       7. The fixing device according to  claim 5 , wherein a width of the base pad in the feeding direction is less than a width of the support member in the feeding direction. 
     
     
       8. The fixing device according to  claim 5 , wherein the base pad satisfies relationships h 1 ≦h 3  and h 2 ≦h 3 ,
 wherein h 1  represents a height of an upstream end portion in the feeding direction of the base pad from one of the nip portion and a virtual extension thereof, h 2  represents a height of a downstream end portion in the feeding direction of the base pad from one of the nip portion and the virtual extension, and h 3  represents a maximum height of a central portion of the base pad. 
 
     
     
       9. The fixing device according to  claim 1 , wherein the support member has an upstream end portion and a downstream end portion in the feeding direction which directly face an inner circumferential surface of the fixing rotary member. 
     
     
       10. The fixing device according to  claim 1 , wherein the support member has a recessed portion, and the heating source is disposed within the recessed portion. 
     
     
       11. The fixing device according to  claim 1 , wherein the rotary fixing member comprises one of an endless belt and an endless film. 
     
     
       12. The fixing device according to  claim 1 , further comprising a reflector disposed between the heating source and the support member. 
     
     
       13. The fixing device according to  claim 1 , wherein the support member comprises a linear base portion that supports the nip forming member and two arms extending substantially perpendicularly from the base portion on a side of the support member away from the nip forming member. 
     
     
       14. The fixing device according to  claim 13 , wherein the arms of the support member each comprise distal tips and proximal base ends attached to the base portion of the support member,
 wherein the tips of the arms are more widely spaced apart than the base ends. 
 
     
     
       15. The fixing device according to  claim 1 , wherein the support member has a substantially W-shaped form in cross-section. 
     
     
       16. An image forming apparatus comprising:
 an image forming unit to form an image on a recording medium; and 
 a fixing device according to  claim 1 , to fix the image on the recording medium. 
 
     
     
       17. The fixing device according to  claim 1 , further comprising:
 rotatable guides disposed at each lateral end of the fixing rotary member to contact the fixing rotary member without extending under the fixing rotary member, the rotatable guides restricting movement of the fixing rotary member in an axial direction. 
 
     
     
       18. The fixing device according to  claim 1 , wherein the nip forming member comprises a base pad which determines a shape of the nip portion, the base bad comprising:
 a contact portion in contact with the opposed rotary member via the fixing rotary member; and 
 an extended portion not in contact with the opposed rotary member and extending upstream in the feeding direction from the contact portion, and over which the fixing rotary member slides, 
 wherein the contact portion and the extended portion are disposed on a same plane. 
 
     
     
       19. The fixing device according to  claim 1 , wherein the nip forming member comprises a base pad which determines a shape of the nip portion, the base bad comprising:
 a contact portion in contact with the opposed rotary member via the fixing rotary member; and 
 an extended portion not in contact with the opposed rotary member and extending upstream in the feeding direction from the contact portion, and over which the fixing rotary member slides, 
 wherein the contact portion and the extended portion each include a recessed curved surface recessed radially inward from the fixing belt. 
 
     
     
       20. A fixing device which conveys a recording medium on which an unfixed image is carried and fixes the image onto the recording medium, comprising:
 an endless rotary fixing belt; 
 a base pad disposed inside a loop of the fixing belt; 
 an opposed rotary member to contact the base pad via the fixing belt, to thereby form a nip portion between the fixing belt and the opposed rotary member; and 
 a heater to directly heat the fixing belt in a region which is away from the nip portion, from inside the loop of the fixing belt, without contacting the fixing belt and without directly heating the base pad, 
 wherein the base pad includes, from downstream in a rotation direction of the fixing belt, 
 a first surface to form the nip portion; 
 a second surface disposed upstream from the first surface and upstream of an entrance to the nip portion in the belt rotation direction; 
 a curved third surface disposed upstream of the second surface in the belt rotation direction, and 
 wherein the second surface is fiat, and 
 wherein the fixing belt is away from the third surface when the fixing belt is not rotating. 
 
     
     
       21. The fixing device according to  claim 20 , wherein the first surface is flat. 
     
     
       22. The fixing device according to  claim 20 , wherein:
 the opposed rotary member includes an elastic layer, and 
 the base pad includes a material having a higher rigidity than a rigidity of the elastic layer of the opposed rotary member. 
 
     
     
       23. The fixing device according to  claim 20 , wherein the base pad includes a friction sheet disposed on a surface thereof opposite the fixing belt. 
     
     
       24. The fixing device according to  claim 20 , wherein the heater is disposed directly opposite an inner circumferential surface of the fixing belt. 
     
     
       25. The fixing device according to  claim 20 , wherein the third surface includes a convex shape facing toward an inner surface of the fixing belt. 
     
     
       26. The fixing device according to  claim 20 , further comprising:
 a support to support the base pad, 
 wherein the support has an upstream end portion and a downstream end portion in the feeding direction which directly faces an inner circumferential surface of the fixing belt. 
 
     
     
       27. The fixing device according to  claim 26 , wherein the support has a recessed portion, and the heater is disposed within the recessed portion. 
     
     
       28. The fixing device according to  claim 27 , wherein a width of the base pad in the feeding direction is less than a width of the support in the feeding direction. 
     
     
       29. An image forming apparatus comprising a fixing device according to  claim 20 . 
     
     
       30. The fixing device according to  claim 20 , wherein:
 a portion of the first surface corresponds to the entrance to the nip portion, and 
 the second surface and the portion of the first surface are on a same surface. 
 
     
     
       31. The fixing device according to  claim 20 , further comprising:
 a support system to support the base pad, 
 wherein the support system includes: 
 an upstream side wall; 
 a downstream side wall opposite to the upstream side wall; and 
 a bottom wall connecting the upstream side wall and the downstream side wall, the bottom wall supporting the base pad. 
 
     
     
       32. The fixing device according to  claim 26 , wherein:
 the fixing belt includes two ends which are lateral ends, 
 the fixing device further comprising: 
 a slip ring disposed at each of the two ends of the fixing belt to protect the two ends of the fixing belt, 
 wherein the slip ring is rotatable. 
 
     
     
       33. The fixing device according to  claim 26 , wherein:
 a contact width between the base pad and the support is smaller than a width of the base pad. 
 
     
     
       34. The fixing device according to  claim 26 , wherein:
 the base pad includes a plurality of projecting portions each projecting toward the support. 
 
     
     
       35. The fixing device according to  claim 26 , further comprising:
 a sliding sheet interposed between the base pad and the fixing belt. 
 
     
     
       36. The fixing device according to  claim 26 , wherein:
 the base pad includes liquid polymer.